SVGHMI: Added clipping to min and max value for all widget using @path,min,max syntax. Added overshot() and undershot() methods to widget base class, called when value is limited to max or min respectively.

SVGHMI: decoupled operation string evaluation from HMI variable uptdate in change_hmi_variable(), paving the way for min/max boundaries enforcement

SVGHMI: Added parsing of min and max value that can be given as @path,min,max in widget description